1. Field of Disclosure
This disclosure relates generally to honeycomb core stiffened composite structures and a method for making the same and, more particularly, to a method for making such structures that allows for fluid flow from cells of a honeycomb core during a cure process. Advantageously, the method of the present disclosure may be used to effectively bond the components of such structures using heat and vacuum, and not super atmospheric pressure, which eliminates the need for bonding the components in an autoclave.
2. Description of Related Art
Honeycomb core sandwich panels or composite structures, which typically comprise composite laminate skins co-cured with adhesives to the honeycomb core, are widely used in the aerospace applications, among others, because of their high stiffness-to-weight (i.e., “specific stiffness”) and strength to weight (i.e., “specific strength”) ratios.
Honeycomb core composite structures may be fabricated utilizing various composite forming methods. The most commonly employed technique involves the use of a vacuum bag molding assembly wherein an impervious membrane or “vacuum bag” is employed for consolidating the composite skins or layers and ensuring proper adhesion thereof to the centrally disposed honeycomb core. More specifically, the lower or base composite skin, the honeycomb core, and the upper or face composite skin are sequentially laid in a rigid mold member so that the honeycomb core is overlaid or covered by the upper and lower composite skins. The upper and lower composite skins are typically formed from uncured “prepreg” or “B-stage” laminates that comprises a fiber reinforcement such as graphite, aramid, or fiberglass fibers (e.g., linear, weaves, or both) disposed in a binding polymeric matrix such as epoxy, phenolic, or other similar organic resinous material. Film adhesive typically forms the bonds between the upper and lower composite skins and the honeycomb core. A vacuum bag is disposed over the rigid mold member and seals thereto thereby forming a mold cavity that is occupied by the uncured/unbonded composite lay-up. The mold cavity is then evacuated to subatmospheric pressure within the mold, superatmospheric pressure is applied to the exterior (in an autoclave oven), and the temperature of the composite lay-up is increased while in the autoclave oven to cure the lay-up. The combination of subatmospheric internal pressure and superatmospheric external pressure tend to consolidate the composite skins, remove air and volatiles from the resin binder, and apply the necessary compaction pressure to ensure full and uniform adhesion of the lay-up.
The use of an autoclave during the formation of the composite structure may, in certain circumstances, be less than desirable. For example, the use of superatmospheric pressure on the composite during the cure is more likely to result in shifting, distortion, or both of the honeycomb core (typically near the edge) in the direction generally transverse to the cells. Additionally, using superatmospheric pressure during the formation of relatively large structures (e.g., having a size, e.g., a length and width, that is greater than about 1 m2) requires unusually large and costly autoclaves. Without superatmospheric pressure, it has been difficult, if not impossible, to produce structures with sufficient strength and ruggedness because of inadequate compaction of the laminate layers, inadequate bonding of the honeycomb core and skins, or both. This is believed to be due, at least in part, to the increased gaseous pressure within the structure during the process of curing bonding, or both. Specifically, in addition to the expansion of the air in the cells according to universal gas equation (PV=nRT), the evolution of volatiles from, for example, the polymeric matrix of the lay-up or absorbed moisture within the honeycomb core, tend to increase the pressure within the cells, which tends to separate the skin(s) and the honeycomb core. Thus, a need exists for a method of forming honeycomb composite structures, especially those greater than about 1 m2 in size, that does not require the application of superatmospheric pressure and still provides for adequate bonding of the skin(s) and the honeycomb core.